The api is just for ammonia, nitrite, nitrateIf your just doing fish only, you can probably get by with the RO Buddy and API. If you’re going to get corals other than maybe some zoas, gsp, or other low light softies that don’t mind less than stellar water quality, I’d get an RODI and a better test kit for at least the big 3 (Alk, Ca and Mg).